This invention relates generally to measurement of surface stresses. More particularly, this invention relates to in-situ measurement of residual surface stresses.
Residual surface stresses have been shown to play a critical role in the stress corrosion cracking of many metal or polycrystalline components in a wide array of devices, such as power plant components (e.g., condenser tubes, steam generator U-bends, expansion transitions, girth welds, etc.). One generally reliable technique to measure residual stress in polycrystalline materials is based on diffraction. The diffraction technique exploits the fact that when a metallic crystalline material is stressed, the elastic strains in the material are manifested in the crystal lattice of its grains. The stress, applied externally or residual within the material, if below its yield strength, is taken up by interatomic elastic macro strain that is spread over several tens of grains. As a result of stress, the distance between lattice planes (i.e., the d-spacing) and the angle (2xcex8) at which radiation is diffracted are changed. That is, diffracted radiation peak position shifts and therefore the elastic strain experienced by the specimen can be quantified.
Generally, there are two types of radiation that have been applied to stress measurement by diffraction, namely, x-ray and neutron radiation. Unfortunately, neutron radiation does not provide for in-situ or surface measurement. Further, existing x-ray diffraction instruments are generally too bulky for good portability to be applied in-situ.
Pennsylvania State University has developed a fiber optic based x-ray detector technology, called the Ruud-Barrett Position Sensitive Scintillation Detector (PSSD). The PSSD uses two independent detection surfaces to collect data from two positions on a diffracted x-ray ring simultaneously, thus, providing a unique capability of precision stress measurement by the single exposure technique. The single exposure technique is based on the fact that a single incident x-ray beam is diffracted at a given xcex8 angle or small range of xcex8 angles, such that a cone of diffracted beams is formed. A plane perpendicular to the cone axis intercepts the cone as a circle when the specimen surface is unstressed, and as a distorted ellipse, when the specimen surface is stressed. The distortion of the ellipse is a measure of that surface stress.
The PSSD uses flexible coherent fiber optic bundles about one meter in length and 3 millimeters by 12 millimeters in cross section to conduct an optical analog of the diffracted x-ray pattern to an electronic component of the PSSD where the pattern is amplified and digitized. Optical signal is produced by the diffracted x-ray pattern striking a scintillation coating where the x-rays are converted to light. This light analogue of the x-ray pattern is transported via the flexible, coherent fiber optic bundles to the electronic component of the PSSD that is usually one or more meters away from the specimen. In the electronic component of the PSSD, the optical signal is amplified by an image intensifier, then converted into a digital electronic signal by diode arrays. The digital electronic signal is then used by a computer to calculate the stress on the specimen surface.
Presently the PSSD instruments are limited to insertion in pipes no smaller than 10 centimeters in diameter and requires that a 200-watt x-ray tube be brought in close proximity (e.g., 4 centimeters) of the surface to be measured. The present PSSD faces several barriers to further miniaturization and improved portability. First, the cross section of the bundle is too large to be used for inside specimen with diameters less than about 7 centimeters even if the size of the x-ray source is significantly reduced. Second, if the fiber optics were reduced in cross-section, the spatial resolution of the diode arrays presently used may be too coarse to provide adequate precision of x-ray stress measurement. Additional information about the PSSD is disclosed in U.S. Pat. Nos. 4,686,631 and 5,148,458. These patents are hereby incorporated by reference for all purposes.
Based on the foregoing, a need arises for an improved PSSD that is easily portable and is capable of measuring stresses between two parallel plates that are a small distance (e.g., about 5 centimeters) apart.
An apparatus for in-situ measurement of residual surface stresses comprises a compact x-ray tube and a detector. In an exemplary embodiment, the detector includes a tube sleeve coupled to the compact x-ray tube, a first fiber optic bundle coupled to the tube sleeve, a second fiber optic bundle coupled to the tube sleeve, and a charged coupled device coupled to the fiber optic bundles. An alternative embodiment consisting of incorporating the fiber optic holders and colimator into the x-ray tube body is also envisioned. X-rays emitted by the x-ray tube are diffracted from a specimen surface and intercepted by the first fiber optic bundle and the second fiber optic bundle. The intercepted x-rays are converted into light and transferred by the first fiber optic bundle and second fiber optic bundle to the charged coupled device. Intensities of the received light are detected and digitized to generate a first ring and a second ring. The residual stress in the specimen surface is calculated based on a difference between the radii of the first ring and the second ring.
In an exemplary embodiment, the detector further comprises scintillators attached to the front end of the first fiber optic bundle and the second fiber optic bundle for converting the x-rays into light. In one embodiment, the scintillators are rare earth scintillators. In another embodiment, the scintillators are cadium zinc scintillators.
In another exemplary embodiment, the detector further comprises an image intensifier positioned between both the first fiber optic bundle and the charged coupled device and the second fiber optic bundle and the charged coupled device. The image intensifiers amplify light received from the first fiber optic bundle and the second fiber optic bundle, respectively.
In yet another exemplary embodiment, the detector further comprises a collimator attached to the tube sleeve for collimating x-rays emitted from the x-ray tube onto the specimen surface.
In an exemplary embodiment, the compact x-ray tube is less than 1 inch in diameter, generates 20 kilovolt x-rays, and requires less than 100 watts. In another exemplary embodiment, the first fibers bundle and the second fiber optic bundle comprise fibers each of approximately 10 microns in size and each bundle has a cross section of approximately 6 mm by 6 mm.
In one embodiment, the first fiber optic bundle is split into two bundles to fit along the compact x-ray tube and is rejoined behind the cathode end of the compact x-ray tube. In another embodiment, the first fiber optic bundle is contoured to fit closely near the compact x-ray tube.
In an exemplary embodiment, the charged coupled device comprises a plurality of pixels and each pixel is capable of detecting photons striking the surface of that pixel. In one embodiment, residual stress on the specimen surface is calculated by a computer attached to receive digitized data from the charged coupled device.
A method for in-situ measurement of residual surface stresses comprises the steps of intercepting a first segment of x-rays diffracted from a specimen surface, intercepting a second segment of x-rays diffracted from the specimen surface, converting the segments into light, detecting intensities in the light from the first segment via a charged coupled device, detecting intensities in the light from the second segment via a charged coupled device, defining a first and second ring based on the intensities detected and a common center calculating a first radius of the first ring, calculating a second radius of the second ring, calculating a difference between the first radius and the second radius, and calculating residual stress in the specimen surface based on the difference.
In an exemplary embodiment, the step of converting the segments into light includes the steps of converting the first segment into light via a first scintillation coating and converting the second segment into light via a second scintillation coating. In another exemplary embodiment, the method further comprises the steps of amplifying the light converted by the first scintillation coating via an image intensifier and amplifying the light converted by the second scintillation coating via an image intensifier.
In another exemplary embodiment, the step of defining a first ring includes the steps of fitting the intensities detected by the charged coupled device into a set of curves, determining apexes in the set of curves, and connecting the apexes to define the first ring.
In yet another exemplary embodiment, the step of defining a second ring includes the steps of fitting the intensities detected by the charged coupled device into a set of curves, determining apexes in the set of curves, and connecting the apexes to define the second ring.